Fire detectors are used for early recognition of fires in areas having a corresponding fire load, to protect people and material assets. Fire detectors are typically mounted on the ceiling and have a round, white housing having a diameter of approximately 10 cm and a height of approximately 7 cm to 10 cm. Because of its function, the housing is seated on the ceiling. Fire detectors are industrially mass-produced products and fire detectors of one model and manufacturer typically appear identical. The corresponding housing size may normally be immediately recognized on the ceiling as a fire detector in spaces having public traffic. The uniformity of mass production and the housing shape therefore always represent a compromise between the function of the fire detector and the taste of the user and the visual demands of the surroundings.
Scattered light fire detectors are therefore predominantly mounted on ceilings, because the smoke is first transported to the ceiling due to the thermal generated by the fire and then propagates along the ceiling. This has the disadvantage that the installation, maintenance, and function testing of the fire detector must be performed on the ceiling. At least a ladder and, in taller rooms, even a lift platform is necessary for these activities. This results in a high outlay in time and money for the cited activities. Therefore, it is desirable to be able to perform the maintenance and regular function testing of the fire detectors using testing devices mounted on a long rod. Furthermore, a defective fire detector is also to be easily replaceable using a tool insert mounted on a rod. For this reason, nearly all fire detectors are inserted into a base to which the necessary supply and transmission lines are permanently connected. Contacts are located in the base, via which the fire detector is connected to these lines. Even when a fire alarm system is first put into operation, the fire detector is to be mountable in the base with the aid of a tool insert of this type, since often the installation and wiring of the base is performed by another company and, sometimes, a long time before the installation of the fire detector itself. Thus, a ladder and/or lift platform is only necessary for wiring the base. The fire detector is predominantly installed in the base, as described in WO 97/05586, for example, by inserting the fire detector in a specific orientation in the base and attaching it using a rotational movement, like a bayonet closure. Therefore, the tool insert used on the installation tool for installing the fire detector in the base is tailored to the shape of the fire detector, in such a way that the frictional force required for the rotational movement is applied via a form fit.
A scattered light smoke detector, which has a light transmitter and a light receiver which are positioned in such a way that a scatter point outside the scattered light smoke detector is situated in the open air, the scattered light smoke detector having a cover for protecting the light transmitter and the light receiver, as well as means for differentiating between smoke and other foreign bodies located in an area around the scatter point, is described in German Patent Application No. DE 101 18 913 A1. The means for differentiating between smoke and other foreign bodies has a processor for analyzing the variation over time of received signals of the light receiver, the processor being connectable to the light receiver. The technology for recognizing a fire employed in this scattered light smoke detector allows an installation of the scattered light smoke detector generally flush with the ceiling. An important step for unobtrusive mounting of fire detectors of this type has thus already been taken. Such a fire detector which is insertable flush with the ceiling requires, however, that it generally only has a flat or only slightly curved and smooth surface, which is formed by a cover disk which covers the fire detector. Therefore, it is extraordinarily difficult to install fire detectors of this type in a base using a rotational movement. Only comparatively low frictional forces may be transmitted via the smooth, flat surface of the fire detector, much lower frictional forces than via a form fit, which is typical in the current fire detectors. In the worst case, the frictional force which may still be applied is no longer sufficient to engage the fire detector contacts with the base contacts. Since the fire detector now terminates flush with the ceiling after being inserted into the base, it is additionally very probable that the mounting tool will slip along the ceiling during the required rotational movement and mark or even damage it.
Furthermore, a scattered light smoke detector which has two light receivers or an imaging lens for a light receiver to set a defined measuring volume is described in German Patent Application No. DE 101 18 913 A1.